ADAU1401 crystal oscillation

The ADAU1401 is used in car infortainment project together with NXP tuner. The main problem is from the DSP Crystal frequency, as long as the crystal oscillates it will affect the FM freq at 98.3MHz. I am using 12.288MHz as MCLK. Any alternate way to resolve this? Just to avoid the FM scan frequency from false stop because of crystal harmonic.

 

We cannot change the threshold on this particular frequency, because certain location or state, some station frequency are using this
98.3MHz frequency. If increase the threshold then this station may not stop during scan.

 

Below Clock Oscillator can create harmonics frequencies to interference to FM:

1       3.072Mhz           x29th  x30th  x31th  x32th  x33th  x34th x35th (start up too slow)

2.      11.2896Mhz        x8th  x9th

3.      12.288Mhz          x8th

4.      18.432Mhz          x6th

5.      24.576Mhz          x4th

 

We have tried to add some RC in the board and have tried increasing the R to a value that the processor does notwork but there is no impact or improvement. Take note that the affected harmonics is roughly at 30uV level but our radio sensitivity is down to 10uV.There must be significant
reduction in the affected harmonics from the DSP itself to be able to bring down its level to very low so that our tuner will not lock onto it but onto the externally received broadcasting station at 98.3MHz.

  • I am not a professional at all, but may be good shielding of the DSP will help?

  • 0
    •  Super User 
    on Mar 21, 2014 3:16 AM

         Hello keddy,

         A measured ADAU1701 EMI spectrum appears below:

    We see the crystal’s 12.288 MHz fundamental frequency at A, but virtually no second or third harmonic at B and C respectively.   That’s because the crystal’s waveforms are nearly sinusoidal, not rich in harmonics. There’s a huge blip at 49.15 MHz (D) – this is the MIPS rate at which this DSP is performing its calculations (1024 instructions  x  48K samples per second).  Lots of internal transistors switch at this rate, generating noise at this frequency and its second harmonic (E) at 98.3 MHz.  This signal is strong enough that sometimes I use a FM radio to find out if the DSP is running!  What can be done about it?

    • Skfir’s suggestion to apply shielding makes sense.  Likely the RF circuits in your FM receiver are already shielded; placing a metal shield (such as those commonly used with ARM processor ICs) over the -1401 and surrounding components would help.
    • SIgmaDSPs produce less EMI to begin with when the PCB layout carefully follows the Best Practices advice at http://ez.analog.com/docs/DOC-1889. A comparison of two prototypes at http://ez.analog.com/message/57200#57200 illustrates the wisdom of observing these recommendations.
    • It may be possible to move the second MIPS harmonic out of the FM band as described next:

         Assuming your PLL is set for the 256xFs mode, substitute for the usual 12.288 MHz crystal, a 13.56 MHz crystal -- this slightly overclocks the DSP to a MIPS rate of 54.24 MHz.  Its second harmonic, 108.48 MHz, is now out of your way.  According to the data sheet, a MCLK rate of 13.56 MHz, corresponding to a period of 73.7nS, just squeaks by the rated 73nS minimum period:

    This crystal change will have your -1401 actually running at a sample rate of 52.97 KHz instead of 48 KHz. To compensate for this, reduce the set frequencies of filters, etc. in your project, multiplying them by 0.9062.  Finally, adjust your ADC input resistors following Page 20 of the ADAU1401 data sheet.

    You could instead decrease the crystal frequency enough to slide that second harmonic below 87.5 MHz – this underclocks the DSP, so there’s much more margin in this direction.  Going this way, you may need to choose the 2x program length (about 500 instructions max at nominally 96KHz) in order to keep your actual sample rate high enough to cover the full 20KHz audio range.

         Best regards,

         Bob

  • Hi Bob,

    Cool ideas! I have some additional information that might prove helpful when using this approach.

    In both cases (overclocking or underclocking), a good way to change the frequency-dependent parameters (filters, timers, and so forth) in you project is to follow this guide: FAQ: How do I change the sample rate of my SigmaStudio system?

    By following the instructions in the section labeled Setting "Non-Standard" Software Sample Rates, you can very easily change the sample rate of your system to any arbitrary frequency, without having to go in and manually change any of the filter settings in the project.

  • 0
    •  Super User 
    on Mar 24, 2014 5:03 AM

         Hi Yagami,

         Much easier!  I thought I had seen something about non-standard sample rates, but I couldn't find it.

         Best regards,

         Bob

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:36 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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